Human bocavirus 1 (HBoV1), an autonomous human parvovirus, causes acute respiratory tract infections in young children. HBoV1 is unique among all small DNA viruses in that it expresses a noncoding RNA of 140 nucleotides from the 3' noncoding region after the capsid protein-coding region. This bocavirus-encoded small RNA (BocaSR) is exclusively expressed in the nucleus and plays an important role both in expression of viral nonstructural proteins and in viral DNA replication. HBoV1 infects and replicates in terminally-differentiated human airway epithelium cultured at an air-liquid interface (HAE-ALI), mimicking natural infection of human airways. Importantly, HBoV1 genome replication in the nondividing airway epithelial cells utilizes error-free Y- family DNA repair DNA polymerase (Pol) ? and Pol ?, and BocaSR specifically interacts with viral nonstructural proteins. On the other hand, HBoV1 is a helper virus for the replication of adeno-associated virus (AAV) in HAE-ALI, and BocaSR also plays a key role in facilitating AAV Rep gene expression and DNA replication. Our central hypothesis is that HBoV1 has evolved to express a noncoding RNA to facilitate expression of the viral nonstructural proteins and their function in licensing viral DNA replication, which mimics a DNA repair process driven by error-free Pol ? and Pol ?. In this application, we aim to reveal the mechanisms underlying the BocaSR-regulated expression of viral nonstructural proteins and viral DNA replication in the nucleus, and to understand the viral manipulation of the error-free Pol ? and Pol ? in DNA synthesis. The BocaSR is novel as it is the first one found among all small DNA viruses and differs from other known RNA polymerase III (Pol III) transcribed viral noncoding RNAs by its unique properties in regulating viral gene expression and playing a direct role in viral DNA replication in the nucleus. Studying BocaSR-regulating parvovirus replication will contribute to the understanding of the functions of other viral noncoding RNAs in virus replication. In addition, knowledge in how parvovirus employs the DNA repair machinery for viral genome replication in nondividing cells will help us understand how a small DNA virus overcomes the barrier to replicate its genome outside of the cell cycle. Dissecting the viral and host determinants of HBoV1 genome replication will help the development of therapeutic approaches to prevent acute respiratory tract infections caused by HBoV1. Finally, deep understanding AAV DNA replication will help find a better way to produce recombinant AAV (rAAV) vector and to increase the efficacy of rAAV gene delivery, in particular, to nondividing cells.